The present invention relates to a biaxially oriented polyester film for floppy disc which is excellent in dimensional stability and flatness.
Polyester film is used favorably for various purposes and particularly useful as base film for floppy disc. With progress in densification of floppy disc medium in recent years, a medium has been required to be small in dimensional change under severe environments, particularly small in reversible thermal expansion and irreversible heat shrinkage.
To meet this requirement, a method in which coefficient of thermal expansion of polyester film used as base film is lowered has been generally employed. More specifically, a heat-set biaxially stretched film having an enlarged refractive index or a reduced crystallinity (Reports on Progress in Polymer Physics in Japan, vol. XXIII, 1980). However, in this technique, although a coefficient of thermal expansion can be reduced to a desired extent, heat shrinkage becomes unfavorably large.
For reducing irreversible thermal shrinkage, a method in which a heat-set biaxially stretched polyester film is further subjected to an off-line heat treatment with relaxation is usually employed (Japanese Patent Publication (KOKOKU) 10769(1983) and Japanese Patent Application Laid-Open (KOKAI) Nos. 57-15927(1982) and 59-127233(1984)). However, although a film having a lower heat shrinkage can be obtained by this technique, there has been problems of low production efficiency, such as low workability, which leads to a high production cost and poor flatness of the obtained film. Improvement, therefore, has been required.